As a rule polymer materials, such as plastics or rubbers, cannot be further processed in their pure form. To this effect they have to be mixed with additives, stabilizers and if necessary with fillers. This mixing process can be carried out continuously for example in specially formed extrusion machines, such as double screw extruders, planetary rolling extruders or single screw extruders. In many cases, however, it is recommended to carry out this mixing process on machines, working discontinuously and in batch quantities, and that in particular, if the mixing process is to be extended in time, in order to protect the material. To this effect for example internal mixers working in batch quantities are used. Subsequent to such a mixing process a further mixing process on a mixing mill takes place, on which the mixing process is carried out at a--compared to the mixer--lower temperature and if necessary during reduction of the temperature.
With the continuous and the discontinuous mixing methods the whole material should show constant properties at the end of the mixing process which are narrowly defined. With the continuous mixing method direct access to the material is possible as a rule only at the end of the processing machine, so that a quality control carried out at this place has considerable delays compared with the actual mixing process. In case of mixing the material in kneading machines the actual mixing process is relatively transparent because of the measurement of the actual power taken up by the kneading machine, the measurement of the temperatures and the like. The measurement of special properties of the material, such as viscosity, elasticity etc., however, is not possible during the mixing process. In case of a subsequent mixing process on a roll mill the operator is often able due to his experience to judge subjectively the said material properties, since the material in form of a rolled sheet located on the work roll of the mixing mill is visible. An objective method of measurement working on-line, however, is neither known.
These limited possibilities of the on-line judgement of viscosity and elasticity of the material presents a large problem above all with the processing of rubber, since in this case the raw materials, e.g. natural rubber, are subject to strong fluctuations of the viscosity at the beginning of the mixing process. These different initial viscosities should be reduced to a range as narrow as possible after the mixing process. It is therefore necessary to detect the actual material properties of the material during the mixing process.
There are numerous attempts to draw conclusions from the output data of a kneading machine, i.e. an internal mixer, onto the actual viscosity of the material. Thus it has become known from the EP 0 172 726 B1 to draw conclusions from the actual power draw of the drive of the mixer and from the temperature of the material within the mixer onto the viscosity of the material and above all onto its variation over a period of time. Measuring methods of this type, however, have the drawback that the measured data of the temperature and of the power course are subject to strong fluctuations within the mixer. In this way the power taken up by the drive changes extremely strongly during each rotation of the rotors, and that up to 50%, since such mixers are operated only with a partial filling. During a mixing cycle changes in the power course by up to 220% of the nominal power do occur. If the fluctuations of the power draw during one rotation each are converted by an adequate compensation into a smoothed signal course, the actual values derived herefrom are relatively inaccurate. Especially serious, however, is the insufficient possibility of temperature measurement within the closed mixer. Because of the high viscosity of many materials an appropriate temperature sensor must be very solid and must be dimensioned accordingly. By means of that a comparatively large heat flow can pass from the tip of the temperature sensor to the--as a rule cooled--wall of the mixer. This effect causes errors in the measurement of the temperature, which can be up to .+-.20.degree. C. As a mixer is constantly operated with a partial filling, the temperature sensor is not surrounded with material during the whole mixing cycle. Above all with very high-viscous materials, such as natural rubber, which have a particularly low filling degree of the mixer, this problem comes extremely to the fore.